[strongSwan] How is to view "initial" packets within ikev2 packets by tcpdump ?

[CpServiceSPb .]

I have Ubuntu 14.04 lts (no "x" ) with strongswan installed and set up with
ikev2 connections also.
I need to see what are in outgoing and incoming within ikev2 packets by
tcpdump locally at the server.
That is I need "initial" packet, for example, I ping road-warrior, so I
need see these icmp packets.
How is it possible ?
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